1. Field of the Invention
This inventin relates to a flat display device which replaces a cathode ray tube, and more particularly to a display device which employs optical elements having the memory function.
2. Description of the Prior Art
Heretofore, cathode ray tubes have been mainly used in a television set, the output device of a computer, the display device of a measuring instrument, etc., but flat display devices have been investigated and developed in compliance with the request for, e.g., the miniaturization of the device. These devices are so constructed and operated that a large number of light emitting elements such as electric bulbs, electroluminescence elements, light emitting diodes and discharge elements; optical modulator elements made of liquid crystal or the like; or elements whose optical states are changed by inputs (in this specification, the above-mentioned elements are generically named "optical elements") are principally arranged on a plane in the form of a matrix, and that a picture is displayed by applying electric signals to the individual optical elements and thus changing the optical states of the optical elements constituting a picture frame.
As one of the expedients for obtaining a bright picture with such flat display device, it is known to employ as the optical elements ones which have the memory function as will be described later.
In the case of driving such flat display device, the fundamental driving method has the disadvantage that the number of driving circuits and the number of connections between the driving circuits and electrodes are enormous. By way of example, consider a character display device of 32 characters .times. 8 lines and of 7 .times. 9 dots per character. At this time, the number of driving circuits and the number of connections become close to 300. When the number of driving circuits is so large, the cost rises, and when the number of connections is so large, the reliability lowers.
There has been published a self-transfer type display device which employs optical elements having the memory function in order to enhance the brightness and which adopts means stated hereunder in order to diminish the number of driving circuits and the number of connections. For the sake of simplicity, the optical elements shall be hereinbelow termed the "cells."
FIG. 1 schematically shows the connections between cells and electrodes in a prior-art device. Here, the cells a.sub.w, a.sub.1, a.sub.2, a.sub.3, . . . have the memory function. The cell in each of display devices having hitherto been published has an equivalent circuit in which a capacitor and a discharge element are connected in cascade as shown in FIG. 2a, one in which a resistance and a discharge element are connected in cascade as shown in FIG. 2b, or one in which a resistance exhibited by a discharge element itself (for example, the abnormal glow-discharge region) is employed without using the external resistance in FIG. 2b as shown in FIG. 2c. Alternatively, the cell is an electroluminescence element, a light emitting diode of the P-N-P-N structure, or the like.
As illustrated in FIG. 3, such cell has a hysteresis characteristic in the brightness versus the electric variable value x (for example, voltage amplitude, current amplitude, pulse interval, pulse width, or pulse period). There is a region in which two states B.sub.1 and B.sub.o or more states of brightness exist with respect to a certain value (light emission maintaining value) x.sub.S of the variable value. Here, x.sub.E denotes the minimum value for maintaining the light emission (minimum light emission maintenance voltage), and x.sub.W the minimum value for starting the light emission (minimum discharge starting voltage).
The cell of the self-transfer type display device requires (1) to have the memory property as described above and (2) to have coupling means between the cells. In the following explanation, the discharge cell shown in FIG. 2c will be referred to as such cell. It is a matter of course, however, that the invention is applicable to any cells fulfilling the above-mentioned two conditions (for example, the cells in FIGS. 2a and 2b).
FIG. 4 illustrates a characteristic obtained in such a way that ionization means, for example, coupling holes are provided between the cells, i.e., between a.sub.w and a.sub.1, between a.sub.1 and a.sub.2, between a.sub.2 and a.sub.3, . . . in the prior-art device shown in FIG. 1, and that the values of the discharge starting voltage x.sub.W at which the cells emitting no light are put into the light emission are plotted versus the distance between the cell emitting light and the cell not emitting light. In this case, x.sub.W1 and x.sub.W5 denote the discharge starting voltages of the first and fifth cells, respectively, x.sub.S denotes the light emission maintaining voltage, and x.sub.E denotes the minimum light emission maintaining voltage.
In the panel provided with the coupling means between the cells as described above, every fourth ones of cathodes K.sub.1, K.sub.2, K.sub.3 . . . of the respective cells a.sub.1, a.sub.2, a.sub.3 . . . are connected to cathode lines as shown in FIG. 1. Voltages V.sub.K.phi.1, V.sub.K.phi.2, V.sub.K.phi.3 and V.sub.K.phi.4, V.sub.KW, and V.sub.A of waveforms shown in FIG. 5 are respectively applied to the four phases of cathode lines K.sub..phi.1, K.sub..phi.2, K.sub..phi.3 and K.sub..phi.4, a writing cathode line K.sub.W, and an anode line A. Here, those parts in the voltage waveform V.sub.KW which are indicated by marks X change in dependence on contents to be displayed. First, using the voltage indicated by the mark X, the writing cell a.sub.W is caused to emit light, to perform the writing. Subsequently, using the four phases of cathode pulse voltages V.sub.K.phi.1 .about.V.sub.K.phi.4, the point of light emission is sequentially transferred. In FIG. 5, both E.sub.A1 and E.sub.A2 designate anode voltages. The voltage E.sub.A2 is applied in superposition on the voltage E.sub.A1 in order to facilitate the shift or transition of the discharge (the same applies in the following description). At the time when a desired cell is caused to emit light, the transfer (whose direction is indicated by an arrow in the figure) is stopped and the display is performed. Among the cells in FIG. 1, those a.sub.2, a.sub.3, a.sub.4, a.sub.6 . . . which are hatched do not execute the display even when they emit light, and execute only the transfer. On the other hand, the cells a.sub.1 a.sub.5, a.sub.9, . . . act as the cells for the display (hereinafter, termed "dots") and also effect the transfer action.
According to the self-transfer type display device of FIG. 1 thus constructed, however the cathode electrodes K.sub.N ( N = 1, 2, . . . ) may increase, the number of lead-out electrodes may be 6. In contrast, in the conventional matrix panel, the number of lead-out electrodes increases with the number of cathode electrodes K.sub.N.
In the case of the four-phase drive illustrated in FIG. 1, the interval of the displaying cells in the direction of self-transfer, i.e., the dot pitch (for example, the interval between the cells a.sub.1 and a.sub.5) l.sub.d becomes four times larger than the pitch of the cells (for example, the interval between the cells a.sub.5 and a.sub.6) l.sub.c. The pitch of the cells has its lower limit determined by the precision of fabrication, etc. Regarding, for example, the AC type plasma cell shown in FIG. 2a, the limit of the pitch l.sub.c of the cells is 0.4 mm at the present. For this reason, in the case of the prior-art device which is subjected to the four-phase drive in the electrode arrangement of FIG. 1, the limit of the dot pitch l.sub.d at the display becomes 1.6 mm. In the case where, on account of the response speed of the cells, a dispersion in the characteristics, etc., the number of drive phases of the cathodes need be increased to five or more, the dot pitch becomes still greater.
In the case of performing a character display of the desk top type, a value of 0.7-1 mm is required as the dot pitch l.sub.d at the display. The prior-art self-transfer type display device has therefore been inappropriate for a display device of the desk top type in spite of the fact that it is high in brightness and small in the number of driving circuits as well as the number of connections.